After months of hints and half-denials, Michael Phelps has re-entered the drug testing pool for international competition, the strongest signal yet that he's planning a return to swimming.

Phelps, already widely regarded as the greatest competitive swimmer in history, has not announced that he's ending his retirement or that he plans to swim at the 2016 Olympics in Rio de Janeiro. And his longtime coach Bob Bowman said Thursday that “there are no definite plans to return to competition at this time.” But United States Anti-Doping Agency records posted Thursday indicate that Phelps would be eligible to compete again in 2014.

The swimmer could not be reached for comment, but he told the Associated Press “nothing is set in stone.”

“Michael is just keeping his options open,” Bowman said in a text message. “He has gotten back into some training, and I suggested that it might be a good idea to rejoin the testing pool. That way, he can compete should he ever get to a point where he was ready or interested to do so.”

The Rodgers Forge native retired after winning his record 18th gold medal at the 2012 Olympics in London. But he has worked out with Bowman at the North Baltimore Aquatic Club in recent weeks. And on Thursday, he was spotted at the Arena Grand Prix meet in Minneapolis.

The swimming world had been watching to see whether Phelps would rejoin the drug testing pool. He faced a Nov. 6 deadline to be eligible for next summer's U.S. Nationals. As it turned out, he re-entered the testing pool well before that date, and was tested twice in the third quarter of this year, according to the USADA records posted Thursday.

NBC commentator and former Olympic gold medalist Rowdy Gaines said he takes the news as a strong sign that Phelps will compete again on the world stage.

“More power to him,” Gaines said. “He's a swimmer. That's what he does best, and he's still very young.”

Gaines has little patience for anyone arguing that Phelps could tarnish his legacy with a subpar performance in Brazil. “I have a problem with anyone saying that,” he said. “Who cares what they think? It's up to him, and really, there's no way he can take away from what he's already accomplished.”

Though Phelps won four gold medals and two silvers in London, he did not dominate as he had in 2004 or 2008, the year he set a new standard by winning eight gold medals in eight races. In preparing for the 2012 Games, he sometimes sparred with Bowman, who chastised him for not training as devotedly as he had in earlier years.

Phelps, 28, said he looked forward to sampling life away from swimming. He spent much of the past 15 months traveling the world, playing golf and promoting his swim academies and other products. He ardently followed the Ravens, watching most home games from a private box at M&T Bank Stadium and cheering the team to victory at Super Bowl XLVII.

Though Phelps appeared to enjoy his life of leisure, many in the swimming world, including chief American rival Ryan Lochte, predicted an eventual comeback.

Phelps alternated between downplaying the possibility and teasing his public with open-ended answers.

“Man, people will believe anything that's written, anything that's on TV,” he said in an interview with The Baltimore Sun this summer. “There's nothing in the works with me coming back to swimming. This is a part of my life I'm enjoying. I've never had freedom like this. I live on my own time. I play golf three or four times a week. I wake up whenever I want. I have a few things to do here and there, but mostly my time is mine. I'm not thinking about changing that.”

Bowman also downplayed the possibility over the summer, saying he and Phelps had not discussed it and adding that he would discourage a comeback unless Phelps demonstrated serious dedication.

Nonetheless, Phelps sightings became more frequent at his old training ground in Mount Washington.

In October, Bowman revealed that Phelps had been working out at NBAC periodically, getting himself in better shape.

Phelps also recently renewed his endorsement deal with Subway. The restaurant chain promptly paired him in a commercial with Pele, only the most famous athlete ever from Brazil, the next country to host the Olympics.

The Rio Games would be Phelps' fifth, and he would compete at age 31, pushing ancient for a top male swimmer. Mark Spitz won his last medal at age 22. Matt Biondi won his last at 26.

Attempted comebacks by past champions have produced mixed results.

Phelps' Olympic teammate, Brendan Hansen, retired after the 2008 Games, citing burnout. But after several years off, he returned to qualify for the 2012 Olympics, where he won bronze and gold medals at age 30.

By contrast, Phelps' early rival, Australian Ian Thorpe, failed in his ballyhooed attempt to qualify for the 2012 Olympics after he had retired in 2006. Spitz was no longer competitive when he attempted to return to the pool at age 41.

Both of those champions spent far more time away than Phelps, whose return might not even feel like a comeback to a sporting public that often ignores swimming in the years between Olympics.

Gaines said he walked away from the sport at the end of college but came back nine months later after surveying the best times in the world and realizing he could still compete. Three years later, in 1984, he won three Olympic gold medals. Gaines said he needed only about three months to feel entirely comfortable in the pool again.

He said Phelps shouldn't need long to find his form given the brevity of his time away. “The last time I saw Michael, he looked great,” he said.

The NBC analyst predicted that if Phelps does return, he'll pursue a less arduous schedule, with a focus on relays and shorter events such as the 200-meter individual medley and the 100-meter butterfly.

Regardless of the odds, Phelps has always thrived on trying to achieve the unprecedented.

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